Vehicle steering control system

ABSTRACT

In a &#34;steer-by-wire&#34; vehicle steering control system which steers steerable wheels with a powered actuator according to an input signal obtained from a sensor associated with a steering wheel, to the end of achieving an optimum vehicle response to a steering input, a compensatory transfer function is introduced between the steering wheel and the powered steering actuator. The compensatory transfer function can be obtained by suitable manipulation of the transfer functions of the hardware involved and a desired overall transfer function. The frequency domain compensatory transfer function is transformed into a time-domain input-output relationship, and the steering mechanism is actuated accordingly as a real time operation. By suitable selection of the desired overall transfer function, it is possible to achieve a vehicle response both stable and favorable.

TECHNICAL FIELD

The present invention relates to a vehicle steering control system, andin particular to a "steer-by-wire" vehicle steering control system whichsteers steerable wheels with a powered actuator according to an inputsignal obtained from a sensor associated with a steering wheel.

BACKGROUND OF THE INVENTION

Various electric vehicle steering systems have been proposed which steersteerable wheels with an electric motor according to an input from asteering wheel while taking into account various conditions of thevehicle at the same time, and an example of such "steer-by-wire"steering system is disclosed in Japanese patent publication No.63-332122 filed by the assignee of the present application.

According to such a steering system, the angular displacement of thesteering wheel is detected by a sensor as an electric signal, and anelectric motor is actuated according to this electric signal. Typically,the steering angle of the steerable wheels is increased for a giveninput from the steering wheel when the vehicle is travelling at arelatively low speed, and the steering angle of the steerable wheels isreduced for a given input when the vehicle is travelling at a relativelyhigh speed so that the maneuverability of the vehicle at low speed andthe stability of the vehicle at high speed can be both ensured.Furthermore, the freedom of vehicle design can be much improved becausethere may be no mechanical linkage between the steering wheel and theactuator for actuating the steering mechanism, as opposed to the moreconventional steering system in which the steering wheel is mechanicallycoupled with the steering mechanism. Also, various undesirable problemssuch as an excessive kick-back to the steering wheel, shimmy and juddercan be avoided.

However, according to such a steering system, since the responseproperties of the electric motor and the vehicle response directlyaffect the handling of the vehicle, the operator of the vehicle who isfamiliarized with vehicles equipped with more conventional steeringsystems may have some difficulty adjusting himself to the behavior ofthe vehicle equipped with a powered steering actuator.

BRIEF SUMMARY OF THE INVENTION

In view of such problems of the prior art, a primary object of thepresent invention is to provide a vehicle steering control system whichis equipped with a powered actuator and can produce a desired lateraldynamic response of the vehicle to a given steering input.

A second object of the present invention is to provide a vehiclesteering control system which is equipped with a powered actuator butallows the vehicle operator to operate the vehicle without experiencingany unfamiliar impression.

These and other objects of the present invention can be accomplished byproviding a steering control system for a vehicle, comprising: steeringinput means; steering mechanism for steering steerable wheels; a poweredactuator for actuating the steering mechanism according to a steeringinput signal from the steering input means; sensor means for evaluatinga response of the vehicle to the steering input signal; and compensationmeans interposed between the steering input means and the poweredactuator to modify the steering input signal so that a desired vehicleresponse to the steering input may be obtained. The vehicle response maybe represented either by a yaw rate response or a lateral accleration ofthe vehicle to a steering input.

Thus, by suitable selection of the transfer function of the compensationmeans, it is possible to achieve an overall vehicle response which isboth favorable and stable. The desired response of the vehicle may beeither a constant without any time delay or a first-order time delaytransfer function.

Typically, the transfer function of the compensation means includes asecond-order derivative term, and a first order integral term. To theend of reducing the interferences from the noises and improving thestability of the system, it is also possible to introduce a secondcompensation means which multiplies a first-order integral term to thesecond order derivative term.

BRIEF DESCRIPTION OF THE DRAWINGS

Now the present invention is described in the following with referenceto the appended drawings, in which:

FIG. 1 is a schematic diagram of the overall structure of the vehiclesteering control system according to the present invention;

FIG. 2 is a sectional side view of the steering wheel shaft and otherrelated parts of the vehicle steering control system;

FIG. 3 is a partly broken-away side view of the front wheel steeringunit;

FIGS. 4a through 4c are collectively a flow chart showing the operationof the vehicle steering control system according to the presentinvention;

FIGS. 5a, 5b and 5c are graphs showing the contents of the controltables which are employed in the vehicle steering control systemaccording to the present invention;

FIG. 6 is a graph showing the relationship between the steering angleand the steering reaction force in the vehicle steering control systemaccording to the present invention; and

FIG. 7 is a block diagram showing the basic concept of the presentinvention.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

FIG. 1 shows the structure of the front wheel steering system to whichthe present invention is applied. A steering wheel 1 is attached to theupper end of a steering shaft 2, and the steering shaft 2 is furtherprovided with a pair of potentiometers 3 and 4 for detecting the angularposition of the steering shaft 2, an absolute encoder 5 which isdescribed hereinafter, and a secondary actuator 6 for producing asteering reaction force. A front wheel steering unit 9 incorporating anelectric motor as described hereinafter is provided in a lower portionof a front part of a vehicle body (which is not shown in the drawings)as a primary actuator. The front wheel steering unit 9 includes asteering rod 10 extending laterally of the vehicle body with each endcoupled to one of the front wheels 13 via a tie rod 11 and a knuckle arm12. The motor 46 (FIG. 3) in the front wheel steering unit 9 is drivenby a power unit 15 which is in turn connected to an electronic controlunit (ECU) 17 via an interface 16. This ECU 17 is also connected, viathe interface 16, to a servo amplifier 18 for controlling theaforementioned secondary actuator 6, as well as to one of thepotentiometers 3 and the absolute encoder 5. The outputs from a yaw ratesensor 19 for detecting the yaw rate of the vehicle, a lateralacceleration sensor 25 for detecting the lateral acceleration of thevehicle, four wheel speed sensors 21 each associated with acorresponding one of the front wheels 13 and the rear wheels 20 fordetecting the rotational speed of the corresponding wheels 13 and 20, anincremental encoder 22 associated with the front wheel steering unit 9to detect the position of the steering rod 10 or the actual steeringangle, and a potentiometer 23 also for detecting the position of thesteering rod 10.

A back-up power unit 24 is provided in parallel with the aforementionedpower unit 15 so that the front wheel steering unit 9 may be controlledand actuated even in the case of failure of the power unit 15 byswitching over a switch 26 provided between the two power units 15 and24. The potentiometers 4 and 23 are connected to a back-up ECU 27 whichcontrols the steering of the front wheels 13 in case of failure of themain ECU 17 by controlling and actuating the front wheel steering unit 9via the back-up power unit 24 according to the steering input obtainedfrom the potentiometers 4 and 23.

FIG. 2 is a sectional view of the structure associated with the steeringshaft 2 including the potentiometers 3 and 4, the absolute encoder 5 andthe actuator 6. The steering shaft 2 is rotatably supported by a casing28 via a ball bearing 30, and the casing 28 is in turn securely mountedon the vehicle body via a bracket not shown in the drawings. Drivengears 33 and 34 extending from the potentiometers 3 and 4 supported bythe casing 28 are received in a chamber 32 defined in a part of thecasing 28. The driven gears 33 and 34 mesh with a drive gear 36 formedin the steering shaft 2 via a counter gear 35.

The left end portion of the steering shaft 2 as seen in FIG. 2 or alower part of the steering shaft 2 is provided with a drive gear 37which meshes with a driven gear 39 extending from the absolute encoder 5in a chamber 38 defined in a middle part of the casing 28. Further, theleft end of the steering shaft 2 as seen in FIG. 2 or the lower end ofthe steering shaft 2 is connected to an output shaft 43 of the actuator6 incorporating a harmonic drive reduction gear and a DC motor, via atubular coupling member 41 received in a chamber 40 defined in a lefthand side of the casing 28 as seen in FIG. 2.

The counter gear 35, the driven gears 33, 34 and 39 each consist of agear set combining a pair of gears fixedly and coaxially secured to eachother with a slight phase shift so that the play of the gears may beeliminated and the precision in detecting angular displacements may beimproved.

Therefore, by operating the steering wheel 1, the driven gears 33 and 34for the potentiometers 3 and 4, and the driven gear 39 for the absoluteencoder 5 are driven in such a manner that the steering angle (steeringangle change rate and steering angle acceleration) can be detected. Theactuator 6 applies a steering reaction force to the steering shaft 2 viathe coupling member 41. The potentiometers 3 and 4 are intended forself-diagnosis and emergency purposes, respectively, and the steeringangle is detected by the absolute encoder 5 under normal condition.

FIG. 3 shows a partly broken-away sectional view of the front wheelsteering unit 9 comprising a casing 45 fixedly secured to a vehicle bodynot shown in the drawing and accommodating the steering rod 10 passedtherethrough, the two ends of the steering rod 10 being each coupled toone of the tie rods 11. In a central part of the casing 45 is receivedthe electric motor 46 which comprises a rotor 49 consisting of a hollowrotor shaft 47 rotatably fitted on the steering rod 10 and coils 48wound around the rotor shaft 47.

An end of the rotor shaft 47 on the left hand side in FIG. 3 is in athreading engagement with a tubular ball nut 50 which is rotatablysupported by the casing 45 at a small diameter portion 52 of the ballnut 50 via a ball bearing 53. The other end of the rotor shaft 47 islikewise supported by the casing 45 via a ball bearing 54. The innercircumferential surface of an axially middle part of the ball nut 50 isprovided with a plurality of annular grooves 55, and a plurality ofsteel balls 57 are disposed between the thread formed around the lefthand side of the steering rod 10 as seen in FIG. 3 and the annulargrooves 55 of the ball nut 50 so that a ball screw mechanism may beformed between the ball nut 50 and the steering rod 10.

When the rotor 49 of the motor 46 is rotated, the rotor shaft 47 isrotated, and this rotational movement is reduced in speed and convertedinto a linear axial movement of the steering rod 10 by the ball nut 50.A part of the steering rod 10 adjacent the right end thereof as seen inFIG. 3 is provided with a splined portion 58 which cooperates with acorresponding splined bore (not shown in the drawings) formed in thecasing 45 for prohibiting the rotation of the steering rod 10.

The outer circumferential surface of the right hand side of the ball nut50 is provided with a gear profile serving as a pulser 59 whichgenerates a pulse signal in the incremental encoder 22 mounted on thecasing 45 for the purpose of detecting the displacement of the steeringrod 10 or the actual steering angle from the angular displacement of theball nut 50. A detection rod 60 extending from the potentiometer 23attached to the right end portion of the casing 45 is connected to theright end of the steering rod 10 via an arm 61 so that the actualsteering angle may be detected from the axial displacement of thesteering rod 10.

Now the operation of the present embodiment is described in thefollowing with reference to FIGS. 1, 4a-4c, 5a, 5b, 5c and 6.

Referring to the flow chart given in FIG. 4, the ECU 17 is initializedin step S1, and the steering angle of the steering wheel 1 or thesteered angle of the front wheels 13 and other variables are initializedin step S2. In step S3, a steering angle θ_(H)(n) obtained from thepotentiometers 3 and 4 and the absolute encoder 5, a vehicle speed Vobtained from the wheel speed sensors 21, a yaw rate obtained from theyaw rate sensor 19 and a lateral acceleration obtained from theacceleration sensor 25 are supplied to the ECU 17. "n" is a count whichis incremented for each cycle of program execution.

After a diagnostic routine based on the values obtained by the varioussensors is carried out according to a prescribed procedure to ascertainthe soundness of the system in step S4, a sampling period t or theinterval between each succeeding signal from the various sensors isdetermined according to the time period required for carrying out anentire cycle of program execution. In step S6, coefficients C₂, C₁,C_(O), C_(C) and d₁ corresponding to the vehicle speed V are determinedfrom the tables given in FIGS. 5a, 5b and 5c so that they may be used invarious equations which are described hereinafter. Thereafter, in stepS7, the steering angular acceleration d² θ_(H) /dt² and the steeringangular velocity dθ_(H) /dt are determined as given in the following:

    d.sup.2 θ.sub.H /dt.sup.2 =(θ.sub.H(n) -2θ.sub.H(n-1) +θ.sub.H(n-2)) /t.sup.2                             (1)

    dθ.sub.H /dt=(θ.sub.H(n) -θ.sub.H(n-1))/t(2)

In step S8, it is defined that:

    C.sub.2 =C.sub.2 d.sup.2 θ.sub.H /dt.sup.2           (3)

    D.sub.1 =C.sub.1 dθ.sub.H /dt                        (4)

    P=C.sub.0 θ.sub.H(n)                                 (5)

    I.sub.(n) =exp(-t/d1)I.sub.(n-1) + C.sub.C {1-exp(-t/d.sub.1)}θ.sub.H(n)                       (6)

In step 59 a steering command value δ_(f)(n) is obtained from

    δ.sub.f(n) =D.sub.2 +D.sub.1 +P+I.sub.(n)            (7)

When the Laplace transform variable is given by s and the desiredtransfer function of the steering response is given by f(s), thesteering command value δ_(f)(n) can be obtained by the following Laplacetransform equations:

    δ.sub.f (s)=G.sub.C (s)θ.sub.H (s)             (8)

where G_(r) (s) is the yaw rate transfer function of the vehicle (or thetransfer function of the yaw rate of the vehicle with the input given asthe steered angle of the front wheels) and G_(A) (s) is the transferfunction of the actuator 9 with the input given as the angulardisplacement of the steering wheel 1. If the desired transfer functionof the yaw rate response of the vehicle to the angular displacement ofthe steering wheel 1 is given by f(s), it can be achieved bycompensating the yaw rate response of the vehicle as represented by thetransfer function G_(C) (s) defined by:

    G.sub.C (s)=f(s)/G.sub.r (s)G.sub.A (s)                    (9)

Assuming that the yaw rate gain is purely constant and there is no phasedelay or

    f(s)=K                                                     (10)

and that the transfer function G_(r) (s) of the yaw rate response of thevehicle and the transfer function G_(A) (s) of the actuator areexpressed by the following forms:

    G.sub.r (s)=(b.sub.r1 s+b.sub.r0) /(a.sub.r2 s.sup.2 +a.sub.r1 s+a.sub.r0)(11)

    G.sub.A (s)=1/(a.sub.A1 s+a.sub.A0)                        (12)

where a_(r2) s, a_(r1), a_(r0), a_(A1), a_(A0), b_(r1) and b_(r0) arecoefficients which are mathematical functions of the vehicle speed anddependent on various parameters of each particular vehicle, one canobtain

    G.sub.C (s)=C.sub.2 s.sup.2 +C.sub.1 s+C.sub.0 +C.sub.C /(d.sub.1 s+1)(13)

where the coefficients C₂, C₁, C₀, C_(C) and d₁ can be obtained bysubstituting equations (10), (11) and (12) into equation (9).

In the present embodiment, a first-order filter which can be expressedby 1/(τ_(1s) +1) is multiplied to the second-order term of equations(13) or the term C₂ s² so that the small fluctuations in the angle ofthe steering wheel may not be picked up by the system as a noise and thestability of the straight ahead movement of the vehicle may be improvedwithout affecting the first derivative term, the proportional term andthe integral term, or without affecting the feel of the steeringoperation as experienced by the operator of the vehicle. Or,

    G.sub.C (s)=C.sub.2 s.sup.2 /(τ.sub.1 s+1)+C.sub.1 s +C.sub.0 +C.sub.C /(d.sub.1 s+1)                                            (14)

By converting equation (14) into time domain equations, one can obtainthe aforementioned equations (1) through (4). The coefficients C₂, C₁,C_(O), C_(C) and d₁ of equations (3) through (6) and (14) can be lookedup from tables as given in FIGS. 5a through 5c so that the determinationof these coefficients may be carried out quick enough not to impair theproper control operation. Since the coefficients a_(r2), a_(r1) anda_(r0) are mathematical functions of the vehicle speed, the coefficientsC₂, C₁, C₀, C_(C) and d₁ are likewise given as values depending on thevehicle speed as indicated in FIGS. 5a-5c.

In step S10, a steering angle command value δ_(f)(n) is supplied to theactuator 9 via the interface 16 and the power unit 15.

Steps S11 through S37 are steps for producing a steering reaction forceto the steering wheel 1. In step S11, it is determined whether thesteering angle command value δ_(f)(n) is less than or equal to aprescribed maximum value δ_(flim), or, in other words, whether thesteering angle command value is less than or equal to the largestpossible value. If so, the program flow advances to step S12 and it isdetermined whether the steering angular velocity dθ_(H) /dt is equal tozero or not. If the steering angular velocity dθ_(H) /dt is equal tozero or the steering wheel 1 is held at a fixed angle, the program flowadvances to step S13 where it is determined whether the absolute valueof the steering angle θ_(H)(n) is smaller than a certain small angleθ_(HO). If the absolute value of the steering angle θ_(H)(n) is smallerthan this small angle θ_(HO), a steering reaction force is obtained instep S14 according to the following equation:

    T=M.sub.2 d.sup.2 θ.sub.H /dt.sup.2 +M.sub.1 dθ.sub.H /dt+M.sub.O' θ.sub.H(n) ±M.sub.C                 (15)

where M₂, M₁, M_(O') and M_(C) are constants. In this equation, the termM₂ d² θ_(H) /dt² serves as a term for controlling the resistance of thesteering wheel at the onset of each steering operation by accounting forthe inertia of the motor rotor 49 and stabilizing the movement of thesteering wheel 1, and the term M₁ dθ_(H) /dt serves as a damping termfor preventing oscillatory movement of the steering wheel 1. The termM_(O') θ_(H)(n) facilitates restoration of the steering wheel to itsneutral position, and the term M_(C) counteracts the force arising fromthe dry friction of the various parts associated with the steeringwheel 1. These coefficients may take either positive or negative valuesdepending on the particular steering wheel structure.

If the absolute value of the steering angle θ_(H)(n) is not smaller thanthis small angle θ_(HO) in step S13, a steering reaction force isobtained in step S15 according to the following equation:

    T=M.sub.2 d.sup.2 θ.sub.H /dt.sup.2 +M.sub.1 dθ.sub.H /dt+(M.sub.O' -M.sub.O)θ.sub.H(n) ±M.sub.C       (16)

where M_(O) is also a constant which is smaller than M_(O'), or M_(O)<M_(O').

It means that the stability of the vehicle is improved by increasing thespring term M_(O) θ_(H)(n) of the spring reaction force T and enhancingthe tendency of the steering wheel to return to its neutral positionwhen the vehicle is travelling straight ahead at high speed and thesteering angle is within a prescribed range around its neutral position,and that the force required for steering operation is kept at anappropriately low level when the steering angle is greater than θ_(HO).

It is also possible to determine M_(O) from a table as a mathematicalfunction of θ_(HO) to achieve a highly fine and non-linear property,instead of simply switching over between M_(O) and M_(O') according tothe magnitude of θ_(H) in steps S13, S19 and S22.

In steps S14 and S15, since the steering angular velocity dθ_(H) /dt iszero and so is the friction term M_(C) which changes sign depending onthe direction of the steering operation, equations (15) and (16) areactually given by

    T=M.sub.2 d.sup.2 θ.sub.H /dt.sup.2 +M.sub.O' θ.sub.H(n)(17)

    T=M.sub.2 d.sup.2 θ.sub.H /dt.sup.2 +M.sub.O θ.sub.H(n) +(M.sub.O' -M.sub.O)θ.sub.HO                        (18)

Then, in step S16, the ECU 17 supplies a control signal to the servoamplifier 18 via the interface 16 to actually drive the motor orsecondary actuator 6 for producing the reaction force, and in step S17the variables θ_(H)(n-2), θ_(H)(n-1) and I.sub.(n-1) are reset as givenin the following before the program flow returns to step S3.

    θ.sub.H(n-2) =θ.sub.H(n-1)                     (19)

    θ.sub.H(n-1) =θ.sub.H(n)                       (20)

    I.sub.(n-1) =I.sub.(n)                                     (21)

If the steering angular velocity dθ_(H/dt) is not zero in step S12, theprogram flow advances to step S18 where it is determined whether thesteering angular velocity dθ_(H) /dt is greater than zero. If thesteering angular velocity dθ_(H) /dt is greater than zero, the programflow advances to step S19 where it is determined whether the absolutevalue of the steering angle θ_(H)(n) is less than θ_(HO). If theabsolute value of the steering angle θ_(H)(n) is less than θ_(HO) theprogram flow advances to step S20 where the steering reaction force T isobtained as given in the following.

    T=M.sub.2 d.sup.2 θ.sub.H /dt.sup.2 +M.sub.1 dθ.sub.H /dt+M.sub.O' θ.sub.H(n) -M.sub.C                    (22)

When the absolute value of the steering angle θ_(H)(n) is less thanθ_(HO), the steering reaction force T is obtained in step S21 as givenin the following:

    T=M.sub.2 d.sup.2 θ.sub.H /dt.sup.2 +M.sub.1 dθ.sub.H /dt+M.sub.O θ.sub.H(n) +(M.sub.O' -M.sub.O)θ.sub.HO -M.sub.C(23)

If the steering angular velocity dθ_(H) /dt is determined to be lessthan or equal to zero, the program flow advances to step S22 where it isdetermined whether the absolute value of the steering angle θ_(H)(n) isless than θ_(HO) in the same way as in step S19, and the steeringreaction force T is obtained in step S23 or S24, depending on themagnitude of the absolute value of the steering angle as given in thefollowing before the program flow advances to step S16.

If the absolute value of the steering angle θ_(H)(n) is greater than orequal to θ_(HO), then

    T=M.sub.2 d.sup.2 θ.sub.H /dt.sup.2 +M.sub.1 dθ.sub.H /dt+M.sub.O' θ.sub.H(n) +M.sub.C                    (24)

If the absolute value of the steering angle θ_(H)(n) is less thanθ_(HO), then

    T=M.sub.2 d.sup.2 θ.sub.H /dt.sup.2 +M.sub.1 dθ.sub.H /dt+M.sub.O θ.sub.H(n) +(M.sub.O' -M.sub.O)θ.sub.HO +M.sub.C(25)

If the steering command value δ_(f)(n) is less greater than δ_(flim) instep S11, the program flow advances to step S25 where it is determinedwhether a flag FL which is described hereinafter is "1". If the flag FLis not "1", the program flow advances to step S26 and the currentlymeasured value of the steering angle θ_(H)(n) is substituted into avariable θ_(Hlim). Then, "1" is placed in the flag FL in step S27, andthe program flow advances to step S12. Thus, the largest possible valueof the steering angle can be set up. If the flag FL is "1" in step S25,the program flow advances to step S28 where it is determined whether thesteering angle θ_(H)(n) is sufficiently (or by more than +α) greaterthan θ_(Hlim) ; the program flow advances to step S12 if the steeringangle θ_(H)(n) is not sufficiently greater than θ_(Hlim) and to step 29if the steering angle θ_(H)(n) is sufficiently greater than θ_(Hlim).The cumulative timer t_(k) is reset in step S29, and the maximumsteering reaction force T_(lim) is substituted into the steeringreaction force T in step S30. After the steering reaction force T isactually produced in step S31, t_(k) is added to the timer t_(k) in stepS32. In step S33, it is determined whether t_(k) has reached aprescribed value t_(p) ; the program flow returns to step S30 if theprescribed value t_(p) has not been reached, and advances to step S34 ifthe prescribed value t_(p) has been reached.

Zero is substituted into the steering reaction force T in step S34, andthe steering reaction force T is actually produced in step S35, while instep S36 t_(k) is added to the timer t_(k) in the same way as in stepS32. In step S37, it is determined whether t_(k) has reached aprescribed value 2t_(p) ; the program flow returns to step S34 if theprescribed value 2t_(p) has not been reached and to step S3 when theprescribed value 2t_(p) has been reached. In other words, in steps S25through S37, the steering reaction force T is oscillated between themaximum value and zero so that the operator of the vehicle may sensethat a limit of the steering angle has been reached.

In the above embodiment, the desired behavior of the vehicle wascharacterized or represented by the yaw rate with a zero phase delay anda constant gain, but it is also possible to use other properties such asthe lateral acceleration and the side slip angle.

It is also possible to measure the yaw rate transfer function of thevehicle G_(r) (s) and the actuator transfer function G_(A) (s) fordifferent vehicle speeds by using appropriate sensors such as the yawrate sensor 19 given in FIG. 1, and to determine the coefficients C₂,C₁, C_(O), C_(C) and d₁ of the transfer function G_(C) (s) of equation(14) which will achieve a desired vehicle behavior f(s) by using a Bodediagram and making suitable compensations.

Further, if it is desired that the yaw rate property involves a firstorder delay with respect to the steering angle θ_(H)(n) in the abovedescribed embodiment, or

    f(s)=K/(τ.sub.2 s+1)                                   (26)

where τ₂ is a time constant, one can obtain ##EQU1## where C_(1'),C_(O'), e₁, e_(O), d₂, d_(1') and d_(O) are coefficients which depend onthe vehicle speed.

FIG. 7 summarizes the basic concept of the present invention. Thetransfer function of the steering actuator and the transfer function ofthe vehicle body are obtained as g(s) and h(s), and a desired overallvehicle response is defined as f(s). The actual yaw rate response of thevehicle to a steering input may be made substantially equal to thedesired overall vehicle response f(s) by compensating the uncompensatedoverall response by a compensatory transfer function 1/g(s)h(s).

According to the present invention, a desired vehicle response propertycan be easily obtained, and the driveability of the vehicle as well asthe feel of the steering operation can be improved by controlling theactuator for activating the steering means through the use of transferfunctions which compensate the overall transfer function of the steeringactuator and the lateral vehicle response. Even when modifications aremade to the actuator and/or the vehicle, a desired vehicle response canbe obtained simply by making a corresponding alteration to thecompensatory transfer functions.

Although the present invention has been described in terms of preferredembodiments thereof, it is obvious to a person skilled in the art thatvarious alterations and modifications are possible without departingfrom the scope of the present invention which is set forth in theappended claims.

What we claim is:
 1. A steering control system for a vehicle,comprising:steering input means; a steering mechanism for steeringsteerable wheels; a powered actuator for actuating said steeringmechanism according to a steering input signal from said steering inputmeans; means for evaluating a response of said vehicle to said steeringinput signal; and compensation means interposed between said steeringinput means and said powered actuator for modifying said steering inputsignal based on an output of said evaluating means so that a desiredvehicle response to said steering input signal will be obtained by thesteering control system.
 2. A steering control system according to claim1, wherein said vehicle response evaluated by said evaluating means isrepresented by a yaw rate response of said vehicle to said steeringinput signal.
 3. A steering control system according to claim 1, whereinsaid vehicle response evaluated by said evaluating means is representedby a lateral acceleration of said vehicle to said steering input signal.4. A steering control system according to claim 2, wherein said desiredvehicle response consists of a response with a constant characteristicand which is effected without any time delay.
 5. A steering controlsystem according to claim 2, wherein said desired vehicle responseconsists of a first-order time delay transfer function.
 6. A steeringcontrol system according to claim 1, wherein said compensation means isadapted to modify said steering input signal by processing said signalwith a transfer function including a second-order derivative term, and afirst-order integral term.
 7. A steering control system according toclaim 6, further comprising a second compensation mean which multipliesanother first-order integral term to said second order derivative term.8. A steering control system according to claim 3, wherein said desiredvehicle response consists of a response with a constant characteristicand which is effected without any time delay.
 9. A steering controlsystem according to claim 3, wherein said desired vehicle responseconsists of a first-order time delay transfer function.
 10. A steeringcontrol system according to claim 1, wherein said desired vehicleresponse is a yaw rate with a zero phase delay and a constant gain. 11.A steering control system according to claim 1, wherein saidcompensation means is adapted to directly modify said steering inputsignal.
 12. A steering control system according to claim 1, wherein saidcompensation means includes means for applying a steering reaction forceto said steering input means.
 13. A steering control system for avehicle, comprising:steering input means; a steering mechanism forsteering steerable wheels; a powered actuator for actuating saidsteering mechanism according to a steering input signal from saidsteering input means; means for evaluating a response of said vehicle tosaid steering input signal; and compensation means interposed betweensaid steering input means and said powered actuator for modifying saidsteering input signal based on an output of said evaluating means, saidcompensation means having a transfer function

    G.sub.C (s)=f(s)/G.sub.r (s)G.sub.A (s)

where f.sub.(s) is a desired transfer function of a dynamic vehicleresponse to a steering input signal from said steering input means,G_(r) (s) is a transfer function of said dynamic vehicle response to asteering input from said steerable wheels, and G_(A) (s) is a transferfunction of an output of said powered actuator to said steering inputsignal from said steering input means.
 14. A steering control systemaccording to claim 13, wherein said vehicle response evaluated by saidevaluating means is represented by a yaw rate response of said vehicleto said steering input signal.
 15. A steering control system accordingto claim 13, wherein said vehicle response evaluated by said evaluatingmeans is represented by a lateral acceleration of said vehicle to saidsteering input signal.